1. Field of the Invention
The present invention relates to an apparatus and a method for correcting a DC component generated by an orthogonal converter in a radio telecommunication apparatus.
2. Description of the Related Art
FIG. 1 is a diagram describing local leakage.
In an amplifier for use in a radio telecommunication apparatus employing a direct modulation method, local leakage is generated by a direct current (DC) component (i.e., a DC offset) that is generated at a digital/analog (D/A) converter and a modulator. The local leakage is a spurious wave and therefore must be reduced in order to attain good quality telecommunications. To reduce the local leakage, it is necessary to have a function for providing an offset voltage (i.e., a DC offset correction circuit) so as to cancel a DC offset generated at a converter. The amount of the DC offset generated at a converter is varied in accordance with temperature and the amplitude of input I and Q signals; therefore, it is desirable that the DC offset be cancelled adaptively by updating the parameter of a DC offset correction circuit at the time a radio telecommunication apparatus is operated. Accordingly, an apparatus is implemented in which a central processing unit (CPU) calculates a DC component by using reference signal data or feedback signal data to update the parameter of the DC offset correction circuit adaptively, thereby reducing local leakage even if the temperature and/or IQ amplitude value change.
Two methods are known as conventional methods for correcting a DC offset.
FIGS. 2A and 2B are diagrams describing a reference signal type DC offset correction method.
The correction of a reference signal type DC offset uses a feedback signal that is the result of demodulating the output of an amplifier to an IQ signal by way of a feedback circuit and a reference signal that is a pre-modulation baseband signal. The method involves subtracting the reference signal from the feedback signal, calculating a parameter in a reverse phase by using an error signal that is the result of extracting only a DC offset component of a transmission signal, and updating the parameter of the DC offset correction circuit, thereby removing the DC offset. Prior to this operation, it is necessary to perform a phase adjustment operation on the feedback signal and reference signal in order to match the signal point phase between the reference signal and feedback signal.
Now, let it be assumed that a vector indicating the reference signal is expressed by “Refdt” as shown in FIG. 2A and that a vector indicating the feedback signal is expressed by “Fbdt” as shown in FIG. 2B. The DC offset generated by the converter and/or D/A converter are added to the Fbdt. Further assuming that the phases of the Refdt and Fbdt are matched with each other by the operation of phase adjustment, and subtraction of the Refdt from the Fbdt that has a DC offset added to it obtains a vector indicating the DC offset because the Refdt is a signal in the state of a DC offset not existing. Therefore, the vector indicating the DC offset that is obtained by an arithmetic logical operation is previously subtracted from the Refdt and then D/A conversion and modulation are applied, thereby obtaining an amplifier output that is the result of reducing the DC offset.
FIGS. 3A through 3E are diagrams describing a feedback signal type DC offset correction method.
The feedback signal type DC offset correction method corrects a DC offset by using only a feedback signal. In this method, a CPU evaluates the vector direction of a DC offset and sets a parameter, which comprises discretionary amplitude in the reverse direction to the evaluated vector direction, in the DC offset correction circuit, thereby cancelling the DC offset. Actually, however, the rotation of a local phase on the frequency converter used in a feedback circuit causes a shift between the vector direction of the DC offset evaluated by the CPU and the actual vector direction. The CPU accordingly performs a discretionary correction, examines how much the amount of rotation of the local phase has affected the feedback signal, and evaluates the vector direction of the DC offset in consideration of the value of the effect.
FIG. 3A is a DC vector that is the result of adding a DC offset to a baseband signal vector. FIG. 3B is a transmission signal vector (i.e., an RxDC vector) that is the result of adding a phase rotation φ to the DC vector. A correction vector that cancels the RxDC vector is generated by rotating the RxDC vector by 180 degrees and multiplying it by an arbitrary constant, as shown in FIG. 3C. Then a TxDC vector is obtained as a result of adding the correction vector to the DC vector, as shown in FIG. 3D. A vector that is the result of rotating the TxDC vector in the amount of the phase φ is a second RxDC vector, as shown in FIG. 3E. The second RxDC vector is a vector that is the result of adding the correction vector to the baseband signal, modulating it after a D/A conversion, and then feeding it back. Therefore, the difference between the RxDC vector shown in FIG. 3B and the second RxDC vector shown in FIG. 3E is a vector that is the result of rotating the phase of the correction vector. Then, a comparison between the differential vector and the initial correction vector obtains a phase rotation amount φ. Therefore, the method is such that, when the correction vector is added to the RxDC vector, a correction vector that is the result of adding the phase correction in the amount of the phase rotation is added to the baseband signal. As a result, the DC offset is corrected to some extent. This is followed by changing the magnitude of the correction vector step by step so that the signal points distribute around the origin of the IQ plane, thus resulting in cancellation of the DC offset.
The characteristics of the feedback signal type DC offset correction method are listed as follows:                Not using a reference signal eliminates the necessity of performing a phase adjustment;        if an un-modulated wave and a local leakage are the same frequency, the amplitude of the un-modulated wave is also cancelled because the amplitude of the un-modulated wave cannot be distinguished from a DC offset; and        if the frequency of the local leakage exists in the modulation band of a carrier, this makes it difficult to extract a DC component, resulting in degradation of the correction accuracy.        
The characteristics of the reference signal type DC offset correction method are listed as follows:                A DC offset is extracted from the error component between the reference signal and feedback signal, making it possible to extract a DC component even if the frequency of the DC offset overlaps with the modulation band of a carrier, thereby making the correction accuracy high; and        for a transmission pattern in which zero amplitude occurs frequently at a burst transmission or the like, a phase adjustment cannot be performed normally, and as a result a DC offset component cannot be extracted and therefore a DC offset correction cannot be achieved.        
The above noted DC offset correction methods are detailed in reference patent documents 1 and 2:
Patent document 1: International Disclosure Number WO2005/025167 A1
Patent document 2: International Disclosure Number WO2005/025168 A1